Numerous studies have reported that microwave radiation might influence brain functions [38, 65, 166]. Various methods were developed to investigate changes in brain function-related behavior, electrophysiological activities and BBB permeability. The methods used to study the harmful effects on brain functions induced by microwave radiation are described in the following sections.

Behavior

The behavioral methods used in studies on the biological effects induced by microwave radiation mainly focused on evaluating the function of learning and memory, anxiety, locomotor activity, depression, and excitability.

The Morris water maze (MWM), named after its inventor Richard Morris [167], was the most widely used behavioral method for learning and memory evaluation, especially for rodents [3, 4, 12, 13, 15, 16, 37, 40, 65, 68, 71, 73, 94, 110112, 168170]. In addition, other methods, such as the Y-maze, eight-arm radial maze, and elevated plus maze (EPM), were also used to study the effect induced by microwave exposure on learning and memory [2, 110, 112, 124, 171].

The most common methods to assess anxiety behavior induced by microwave exposure were the EPM and open field test (OFT) [71, 168, 172]. The EPM was designed based on the conflict between a rodent's instinct to explore a novel environment and their preference for closed arms. The EPM was widely used in anxiety assays [173]. As a popular behavioral test, the OFT is appropriate for social rodents with small living conditions. The OFT was used to measure anxiety-like behaviors in studies on microwave-radiation-induced injuries [71, 168, 174].

In addition, the behavioral tests commonly used to assess locomotor activities included the OFT [25, 124, 125], rotarod tests [125], and accelerated rotarod systems [124]. Moreover, forced swimming tests (FSTs) [124, 168] and tail suspension tests (TSTs) [168] were implemented to evaluate the level of depression induced by microwave radiation.

Electrophysiological activities

In studies on brain physiological activity changes caused by microwave radiation, the most commonly used methods were electroencephalography (EEG) and in vivo hippocampal long-term potentiation (LTP) recording. EEG could be used to reflect changes in brain function, including sleep quality [70, 166, 175186]. LTP recording is a well-recognized electrophysiological method used to study synaptic plasticity induced by microwave radiation with respect to learning and memory [13].

BBB permeability

Evans blue (EB) staining was the most popular method used to study the changes in BBB permeability induced by microwave radiation.

Serum albumin is the main serum protein that cannot cross the BBB under physiological conditions. EB dye can bind to serum albumin tightly. Therefore, serum albumin can be traced by EB dye when using fluorescence microscopy. When BBB permeability increases, EB-dye-bound albumin may extravasate through the BBB into extracellular brain tissue [187]. It was reported that more EB dye was observed in brain tissue after microwave exposure [72, 90, 91, 188, 189].

In addition, several methods, such as albumin immunohistochemistry staining [89, 114116, 190], transendothelial electrical resistance (TEER) measurement [38], horseradish peroxidase (HRP) staining method [38], and 14C-sucrose-tracing methods [148], could also be used to study the effects on BBB permeability induced by microwave radiation. TEER indicates the impedance to pass through the BBB, which is recognized as one of the most accurate and sensitive indicators of BBB integrity [38].

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